The invention relates to a method for fabricating a barrier layer which is suitable in particular for the fabrication of multilayer capacitive structures which have oxygen-rich metal oxide layers having high dielectric constants.
In MOSFET transistors, improved capacitive control of the conductive channel located uncler the oxide layer can be achieved by reducing he layer thickness of the gate dielectric or by using dielectric materials having high dielectric constants.
In volatile memory cells, in particular dynamic DRAM memory cells, the storage capacitance of the capacitive memory structures is decreased with increasing miniaturization, with the result that, for compensation, it is necessary to increase the capacitance per unit area or the capacitance per area. This is likewise achieved by reducing the thickness of the dielectric layer or by using dielectric materials having high dielectric constants.
Certain metal oxide layers are distinguished by particularly high dielectric constants. Known metal oxides having high dielectric constants are titanium dioxide, tantalum pentoxide or aluminum oxide, Such metal oxides are fabricated by application of a metal layer and subsequent thermal oxidation thereof. In this case, the metal, such as, for example, titanium, tantalum or aluminum, is deposited by means of various deposition methods such as sputtering, CVD methods or MBE methods, and is subsequently thermally oxidized. Such metal oxide layers cannot, however, be applied directly on the substrate since, during the thermal oxidation of the metal layer, metal ions penetrate into the silicon substrate and form conductive metal silicide compounds which can cause short circuits. Therefore, there must be a barrier layer present between the applied metal layer and the silicon substrate before the metal layer s oxidized to form the metal oxide layer.
Hitherto, a layer made of silicon dioxide below the metal layer has been used as barrier layer. However, such a silicon dioxide barrier layer has the disadvantage that, during oxidation of the metal layer lying above it, the silicon dioxide barrier layer grows into the silicon substrate and is thus widened. The widening of the silicon dioxide layer results in a considerable reduction in the capacitance of the multilayer capacitive structure comprising the metal oxide layer and the underlying silicon dioxide layer.
The object of the present invention, therefore, is to provide a method for fabricating an oxygen-impervious barrier layer which makes it possible to form a capacitive structure with a meal oxide without contaminating the substrate.
The invention provides a method for fabricating a barrier layer having the following steps, namely oxidation of a substrate composed of silicon in order to produce a substrate oxide on the surface of the substrate; production of an oxygen-impervious layer at the interface between the substrate oxide layer and the substrate, the oxygen-impervious layer, as barrier, preventing the formation of metal silicide compounds between applied metal and the substrate silicon; etching of the substrate oxide layer until the underlying oxygen-impervious layer is uncovered.
The method according to the invention affords the advantage that, during the growth of a metal oxide layer on the barrier layer, no oxygen can penetrate through the barrier layer and, consequently, no undesirable silicon dioxide layer which reduces the capacitance can be produces between the barrier layer and the substrate.
In a first embodiment of the fabrication method according to the invention, the oxygen-impervious layer is produced by implanting nitrogen ions into the substrate, the substrate being oxidized in such a way that a substrate oxide layer and an underlying substrate-nitrogen compound as oxygen-impervious layer are formed.
In an alternative embodiment of the method according to the invention, the oxygen-impervious layer is produced by the substrate oxide produced on the surface of the substrate being exposed to a nitrogen-rich gas in such a way that a substrate-nitrogen compound as oxygen-impervious layer forms at the interface between the substrate oxide and the substrate.
In this case, the substrate oxide is preferably exposed to a pure nitrogen gas, an NO gas, an N2O gas or an NH3 gas.
The substrate-nitrogen compound preferably comprises pure silicon nitride.
In a further embodiment, the substrate-nitrogen compound comprises silicon oxynitride.
In a first embodiment, the substrate oxide layer is etched by wet-chemical etching of the substrate oxide layer.
In a further embodiment of the method according to the invention, the substrate oxide layer is etched in a dry etching process.
Preferred embodiments of the method according to the invention are described below with reference to the accompanying drawings in order to elucidate features which are essential to the invention.